1. Field of the Invention
This invention generally relates to methods and devices of chemical analysis of fluids and gases. In particular, utilizing ion mobility techniques for detecting and identifying components of interest in a fluid mixture such as in a formation fluid.
2. Background of the Invention
In the field of chemical analysis the use of ion mobility spectrometers have been widely used. Ion mobility spectrometers separate ionic species based on their ion mobility in a given media (either gas or liquid). For example, several approaches to chemical identification are based on the recognition that ion species have different ion mobility characteristics under different electric field conditions at atmospheric pressure. These approaches include time-of-flight Ion Mobility Spectrometry (IMS) and differential mobility spectrometry (DMS), the latter also known by other names such as field asymmetric ion mobility spectrometry (FAIMS). Ion mobility measurements have been widely used for identification of components including but not limited to drugs, explosives, and chemical warfare agents [Eiceman. G. A., Karpas Z., Ion Mobility Spectrometry, CRC Press, 2005].
In a conventional time-of-flight Ion Mobility Spectrometry (IMS) device, a weak DC field gradient is established between an upstream electrode and a downstream collector electrode and then an ionized sample is released into the DC field. The ionized sample flows toward the collector electrode. Ion species are identified based on the time of flight of the ions to the collector. The DC field is weak where ion mobility is constant. In other words, the IMS spectrometers separate ions based on their steady state ion mobilities under constant electric field. More recently, improvements have been reported in the lower limits of detectability for ion mobility instruments. In U.S. Pat. No. 5,218,203 a device is disclosed for restricting a sample gas from entering the drift region and limiting sample gas ions to such regions. However, there are several limitations of convention IMS spectrometers instruments: first, they require high resolving power for operation; and secondly, the drift tubes used in the IMS devices are still comparatively large and expensive and suffer from losses in detection limits when made small. The search therefore still continues for a successful field instrument that includes both a small ion injector/column and a small detector/spectrometer and yet is able to rapidly produce unambiguous orthogonal data for identification of a detected compound.
A typical differential mobility spectrometry (DMS) device includes a pair of opposed filter electrodes defining an analytical gap between them in a flow path (also known as a drift tube or flow channel). Ions flow into the analytical gap. A compensated high-low varying asymmetric RF field (sometimes referred to as a filter field, a dispersion field or a separation field) is generated between the electrodes transverse the ion flow in the gap. Field strength varies as the applied RF voltage (sometimes referred to as dispersion voltage, separation voltage, or RF voltage) and size of the gap between the electrodes. Also, ions are displaced transversely by the DMS filter field, with a given species being displaced a characteristic amount transversely toward the electrodes per cycle. DC compensation is applied to the electrodes to compensate or offset the transverse displacement generated by the applied RF for a selected ion species. The result is zero or near-zero net transverse displacement for that species, which enables that species to pass through the filter for downstream processing such as detection and identification. Other ions undergo a net transverse displacement toward the filter electrodes and will eventually undergo collisional neutralization on one of the electrodes. Both the typical DMS and IMS devices separate the ions through the use of nonlinear mobility, which occurs at high values of normalized electric field. The normalized electric field refers to the relation between the applied electric field at a given location in space divided by the neutral particle number density. The normalized electric field is a key parameter in ionized gases and plasmas, as the energy of ionized particles, the breakdown and sustaining voltages and other key parameters depend upon this ratio. The DMS devices have sensitivity and selectivity that are still substantially worse (less) than linear drift tubes. Further, such systems typically operate at atmospheric pressure.
However, at least one limitation of convention DMS systems is that the compensation voltage applied to the filter electrodes typically generates fringe fields that force ions to impact and deposit charge along the flow path of the system adjacent to the filter. As the ions deposit their charge, a charge build up occurs that counteracts the influence of the fringe fields and allows for subsequent stable ion detection. Unfortunately, the period of time in which the DMS system reaches stable ion detection introduces response time delays, especially in a system performing multiple sample detections, which may reduce the speed and responsiveness of current DMS systems. Also, the dependence on a charge build up to enable stable ion detection may adversely affect the stability and sensitivity of the DMS system where the charge build up is dependent on other variable factors such as surrounding environmental conditions.
Moreover, in many cases, in a less-than ideal operating surface environments (in particular those with high humidity, temperature or other site-specific interferences), the above noted spectrometers, e.g., IMS, DMS or FAIMS, performance is significantly limited. The performance of the ion mobility spectrometers in these circumstances can be improved by increasing the temperature of the gas. High temperature ion mobility spectrometers are common in applications that require high resolution analysis, such as explosive detection. Unfortunately, the use of high temperature drift tubes in differential mobility spectrometer devices results in high power consumption, limited portability and other operational disadvantages, including slow turn-on from cold conditions. In addition, dry drift gas is often required in these types of spectrometers. A dehumidifier in front of the unit has been used to address these problems (either as a water absorber or as a hydrophobic membrane) with significant trade-offs. The volume and weight, as well as the need for regeneration, makes the use of dehumidifier cell impractical, while the use of the hydrophobic membrane decreases the volume/amount of the sample that is introduced into the device, decreases its sensitivity.
Therefore, there is a need to develop a spectrometer that could overcome at least some of the above noted limitations over the known spectrometers.